A cold crucible induction furnace is used to melt electrically conductive materials placed within the crucible by applying a magnetic field to the material. A common application of such furnace is the melting of a reactive metal or alloy, such as a titanium-based composition, in a controlled atmosphere or vacuum. FIG. 1 illustrates the principle features of a conventional cold crucible furnace. Referring to the figure, crucible 100 includes slotted wall 112. The interior of wall 112 is generally cylindrical. The upper portion of the wall may be somewhat conical in shape to assist in the removal of skull as further described below. The wall is formed from a material that will not react with a metal load placed in the crucible and is fluid-cooled by conventional means. For a titanium-based load, a copper-based composition is suitable for wall 112. Slots 118 have a very small width (exaggerated for clarity in the figure), typically on the order of 10 to 12 thousandths of an inch, and are filled with a thermal conducting, but electrical insulating material, such as mica. Base 114 forms the bottom of the crucible volume that is available for the metal load. The base is typically formed from the same material as wall 112 and is also fluid-cooled by conventional means. The base is supported above bottom structural element 126 by support means 122 that may also be used as the feed and return for a cooling medium. Base 114 is raised above bottom structural element 126 and generally limits the bottom of the induction coil to be above the height of base 114. A layer of a thermal conducting, but electrical insulating material 124 (thickness exaggerated in the figure) separates the base from wall. Typically, but not by way of limitation, the distance of separation is in the range of 0.008-inch to 0.012-inch, but as noted, may be touching, or may be as large as 1/16th of an inch. Induction coil 116 surrounds the wall of the crucible and is connected to a suitable ac power supply (not shown in the figure). When the supply is energized, current flows through coil 116 and an ac magnetic flux-producing field is created. The magnetic flux induces eddy currents in wall 112, base 114 and the metal load placed in the crucible. Flux penetration into the metal load is principally through slots 118 and a thin layer of bounding wall material. Heat generated by the eddy currents in the load melts the load. A portion of the metal load adjacent to the cooled wall and base freezes to form a skull around a molten metal product that is removed from the crucible. After removal of molten metal product from the crucible, the skull is removed from the crucible and can be used as scrap feed for a later melt of the same composition. The amount of heat energy generated in the load relative to the applied electrical energy defines the approximate efficiency of the crucible. Heat generated in the wall and base represent the major losses in the process.
A disadvantage of the conventional cold crucible 100 in FIG. 1 is that the wall-base interface interferes with flux transfer to the load in the vicinity of the interface. As shown in FIG. 1, representative flux line 120 illustrates that in the vicinity of the interface, there is a substantial decrease in magnetic flux penetration into the crucible that limits heating of the load in the region of the interface. This decrease in flux effectively limits the range of metal load capacity that the furnace can efficaciously operate within. For example, the furnace shown in FIG. 1 may provide satisfactory operation when the load capacity is between full and approximately 60 percent capacity, as represented by dashed line 127. Below 60 percent capacity, the quantity of supplied energy and/or process time increases to the point that the melting process becomes extremely inefficient. Consequently, the user of the furnace is severely limited in actual capacity operating range relative to the total capacity of the crucible.
Therefore, there exists the need for apparatus and a method of induction melting with a cold crucible wherein the flux transfer to the metal load in the vicinity of the wall-base interface allows an overall increase in efficiency as well as increasing the potential range of charge capacity of metal loads that can be melted efficiently.